Coupled with increased cloud cover and atmospheric moisture content, minimum temperatures are expected to remain above normal for the most part while maximum temperatures will be in the normal to below-normal range. Over the interior, isolated to scattered thundershowers are expected over the northeastern parts at first, shifting westwards to the central parts by the weekend. There are also early indications of the possible development of an intense upper-air system over the western part by Sunday (21st), which may bring widespread rain and thundershowers to the central interior – the exact intensity and location of rain this far ahead of time is very uncertain. The expected wet conditions are a continuation of widespread rainfall that started by the end of March, favouring especially the central parts of the country including the western maize-production region where rainfall totals for April to date range between 60 and 150 mm.
The following is a summary of weather conditions during the next few days:
• General:
o On average, the central, eastern and southeastern parts will receive above-normal rainfall.
o Both the western and eastern maize-production areas are expected to receive normal to above-normal rainfall.
o The southwestern parts of the country, including the winter rainfall area, will be dry for the most part and may receive below-normal rainfall.
o Temperatures over the interior will on average remain in the normal to above-normal range.
o Temperatures over the southern parts will be below normal.
o There is no indication of early frost given the expected weather conditions during the next few days.
o Showers and thundershowers will remain in place for the most part over the central to eastern areas.
o The southeastern to eastern coastal belt and adjacent interior will experience several days with extensive cloud and showers.
o Fresh to strong southeasterlies will occur over the southwestern parts on Tuesday (16th) and Wednesday (17th) as well as during the weekend.
• Rainfall:
o Scattered thundershowers will occur from Wednesday (17th) to Thursday (18th) over the northeastern parts, reaching as far west as the central parts of North West and northeastern Free State.
o Isolated to scattered thundershowers are possible over the central to southeastern parts by Friday (19th) and Saturday (20th).
o Scattered to widespread rain and thundershowers are possible over the central, southeastern and eastern parts on Sunday (21st) and Monday (22nd).
o There are early indications of possible heavy rainfall over large parts of the Free State and North West by Sunday (21st) and Monday (22nd). This outlook is fairly uncertain given the long lead time.
o Thundershowers may spread into the western interior according to current projections by Sunday (21st) or Monday (22nd).
o Overcast and cool conditions with rain is possible over the central to eastern parts on Sunday (21st) and Monday (22nd).
o Rain and showers are possible along the Garden Route from Sunday (21st).
• Temperatures:
o Maximum temperatures over the western maize-production region will range between 16 and 30 °C while minimums will be in the order of 11 – 18°C. The low maximum temperatures are indicated only by Sunday, associated with rainy conditions according to current forecasts.
o Maximum temperatures over the eastern maize-production region will range between 14 and 27°C while minimums will be in the order of 10 – 15°C. The low maximum temperatures are indicated only by Monday, associated with rainy conditions according to current forecasts.
o Relatively warm conditions are expected by Thursday (18th) to Saturday (20th) over the interior, when maximum temperatures will range between 27 and 33 over large parts.
o Maximum temperatures will fall sharply from Sunday (21st), when large parts may experience maximum temperatures between 14 and 18°C over the interior (according to current forecasts with a long lead time).
Seasonal overview
El Niño and seasonal forecasts
According to the Australian Bureau of Meteorology the chances for the strengthening of El Niño during the next few months are diminishing while according the International Research Institute for Climate and Society, there remains a reasonable likelihood of an El Niño during 2019. In general, both these outlooks are less optimistic for the strengthening of a warm event:
The Southern Oscillation Index has trended positive during the last few days, and is still outside the El Niño threshold.
Based on the weak El Niño, forecast models lean towards a tendency for drier conditions by late summer, following (according to earlier forecasts) a wetter early to mid-summer period over the interior. Coupled with the dry signal towards late summer, there is also a concomitant indication of warmer than usual conditions. The positive temperature anomalies are also indicated for early-to mid-summer. While seasonal forecasts tend to indicate drier conditions towards late summer, this is a weaker signal than what is sometimes associated with El Niño summers, possibly at least in part due to the weakness of the event. Recent seasonal forecasts remain negative with regards to rainfall over South Africa towards late summer.
During late December and early January, it may once again be drier – basically during the period when the mid-summer drought usually occurs. If this dry period develops, it will most likely not be as severe as during 2017/18. From late January, conditions may very well improve again, and then even more so from early February. There is a possibility that large parts of the summer rainfall region could receive normal to above-normal rainfall during February and/or March, while globally the indicators should start signaling the possibility of a La Niña towards 2019/20. Should the wet conditions develop in the north, there is also an enhanced likelihood of tropical systems (such as tropical depressions/storms/cyclones) influencing the region. Maps below...
Above-normal rainfall is more likely to occur over the eastern parts of the summer rainfall region during early to mid-summer (left – OND – October, November, December), while the west is likely to remain drier than normal. Towards late summer (right – JFM – January, February, March), there is a strong indication that above-normal rainfall may develop over the northeastern parts of the country, spilling also into the central parts. The western parts will still be more likely to receive below-normal rainfall. These maps are similar to the maps published previously, but here the tendency for below-normal- and for above-normal rainfall is shown instead of the tendency for normal-to-above-normal rainfall.
Seasonal outlook: Summary
Based on the current state of El Niño, it is safe to assume that there will be a tendency towards drier and warmer conditions at least in part during the summer. However, both Global Coupled Models and forecasts based on the decadal variability in the climate system suggest a very weak negative influence. The only difference here is that the predictions based on decadal variability (issued here) suggest increasing wetness towards the end of the summer, with a drier start, while Global Climate Models suggest wetter conditions earlier, drying somewhat towards late summer.
The atmospheric circulation patterns favored the northeastern parts of the country for near-normal to above-normal rainfall during the period October – December 2018. Frequent anti-cyclonic upper-air conditions as well as persistent westerly winds at the surface kept the central to western parts dry. Most of the forecasts indeed indicated some kind of west – east improvement in expected conditions. The relatively wet area was focused more strongly further south, over the Eastern Cape, in the forecast issued by the IRI. The forecast issued by SAWS had a stronger positive signal over the Lowveld instead of the northern Highveld. The forecast issued in Cumulus also indicated the positive signal in the northeast, somewhat closer to the area where rainfall was near normal to above normal. Drier conditions in the west were resolved fairly well by all three forecasts.
Expected rainy season progression, associated with decadal variability – update
The forecast above (published in Cumulus), issued in October 2018, indicated a drier early part of summer and wetter conditions in the second half of summer, particularly in February and/or March.
The following is a very concise summary of weather patterns over the summer rainfall region, intended to provide an overview of observed conditions in the light of the seasonal forecast:
November 2018 was an extremely hot and dry month over especially the central parts of the country. During December, above-normal rainfall occurred over the northeastern parts, but it remained dry over the central parts for the most part until the 27th. From 27 December to 8 January, widespread rain occurred over large parts of the summer rainfall region.
Mid-summer drought conditions occurred from 10 to 25 January. Wet conditions replaced the dry weather from late January and lasted until about 20 February. Towards the end of this wet period, a tropical low resulted in heavy falls along the northern escarpment.
Another warmer and dry situation then developed from the 20th of February and lasted until 8 March. From 8 March, and especially around the 11th, widespread rain and thundershowers occurred over the interior, focusing especially on North West and the Free State. Most recently, thundershowers (mostly isolated to scattered) located mostly over the western to central parts, moving to the extreme east from today (19 March).
The observations above indicate that the latter part of summer so far was wetter than the early summer, in accordance with the earlier outlook provided in Cumulus, and based on the decadal climate variability signal over South Africa. Together with somewhat above-normal rainfall, tropical system had a direct impact over the extreme north-eastern parts of the country.
Rainfall (% of long-term mean): March 2019
The southern half of the maize-production region received normal to above-normal rainfall while the northern parts received below-normal rainfall during March.
Rainfall (mm): 1 – 10 April 2019
Rain occurred over large parts during early April. The western maize-production region especially received welcome rain.
Vegetation Condition Index: 1 – 10 April 2019
Vegetation activity remains near normal to above normal over most of the maize-production region, especially now over the western parts. This is due to widespread rain in February and April, together with relatively high minimum temperatures over the region.
Overview of expected conditions over South Africa during the next few days
An upper-air trough moving through and a ridging high will result in scattered thundershowers over much of the eastern parts from Wednesday (17th) to Thursday (18th). More significantly, current forecasts indicate the possible development of a cut-off low over the western parts together with a strong ridging Atlantic Ocean Anticyclone from Saturday (21st). This system is expected to result in widespread rain and thundershowers over the central to eastern parts, especially from Sunday (21st). According to current forecasts, there is potential for heavy rainfall over large parts of the Free State and North West on Sunday (21st) and Monday (22nd).
Frequent ridging of the Atlantic Ocean Anticyclone will result in cooler conditions dominating in the south while temperatures over most of the rest of the interior are expected to remain in the near normal to above-normal category. Associated with the high to the south, fresh to strong southeasterlies will dominate in the southwest.
Conditions in main agricultural production regions (16 - 22 April)
Maize production region: The region will continue to receive normal above-normal rainfall during the next few days. This area will warm up until Friday when maximum temperatures over the western parts may approach 30°C. Scattered thundershowers are expected over the eastern to northern parts until Thursday (18th). From Saturday (20th), thundershowers will occur over the western parts. It will become cloudy over most of the region with widespread rain and thundershowers from Sunday, according to current forecasts. There is also a chance of heavy falls over especially the western to southwestern parts of the region according to current forecasts from Sunday onwards. With the expected rainy, overcast conditions from Sunday, maximum temperatures are expected to drop to the middle teens, with lowest temperatures indicated over the Eastern Highveld.
Maximum temperatures over the eastern maize-production region will range between 14 and 27°C while minimums will be in the order of 10 – 15°C. Over the western parts, maximum temperatures will range between 16 and 30°C while minimums will be in the order of 11 – 18°C
Cape Wine Lands and Ruens: This region will be mild to cool for the most part while little to no rain is expected according to current forecasts. South-easterly winds will dominate over the southwestern parts, becoming strong (according to current forecasts) by Sunday and Monday. Light showers are possible over especially the southern parts by Friday (19th) and Sunday (21st). According to current forecasts, the upper-air low over the interior is not expected to impact the region. However, with such a long lead time, the position and intensity of the low is very uncertain.
Possible extreme conditions - relevant to agriculture
• Some thundershowers over the interior may have the tendency to become severe, producing strong gusts and hail:
o Over the central to eastern parts of the Northern Cape and possibly further east, from Saturday.
• Heavy falls (>50 mm in 24 hours) may occur over the western Free State and surrounding areas into North West, the Northern Cape and the Eastern Cape from Sunday. In some areas, where soils are saturates, these may lead to flooding. This forecast has a long lead time and associated greater uncertainty regarding location and intensity of the rain.
• Expected persistent cloudy and rainy weather from Sunday over the grain production areas may be conducive to the occurrence of certain diseases and pests associated with prolonged moist conditions.
• Strong southeasterlies are possible over the southwestern parts until Thursday (18th) and more so from Sunday (21st). These may be conducive to the development and spread of wild fires where vegetation is dry.
